U.S. patent application number 11/320523 was filed with the patent office on 2006-07-20 for membrane electrode assembly for improved fuel cell performance.
Invention is credited to Reto Corfu, Jake De Vaal, Robert M. Holland.
Application Number | 20060159979 11/320523 |
Document ID | / |
Family ID | 36091491 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060159979 |
Kind Code |
A1 |
De Vaal; Jake ; et
al. |
July 20, 2006 |
Membrane electrode assembly for improved fuel cell performance
Abstract
A membrane electrode assembly comprises an ion exchange
membrane, an anode positioned on one side of the membrane and a
cathode positioned on the other side of the membrane so that a
portion of the cathode extends outside of the anode area on the
oxidant outlet side of the fuel cell such that any hydrogen leaked
from the anode side to the cathode side due to any defects (holes)
existing in the membrane near the oxidant outlet is recombined with
the oxygen on the cathode side before it reaches the oxidant outlet
and no hydrogen is present in the oxidant stream exhausted from the
fuel cell.
Inventors: |
De Vaal; Jake; (Coquitlam,
CA) ; Holland; Robert M.; (Richmond, CA) ;
Corfu; Reto; (Vancouver, CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Family ID: |
36091491 |
Appl. No.: |
11/320523 |
Filed: |
December 27, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60639665 |
Dec 28, 2004 |
|
|
|
Current U.S.
Class: |
429/434 ;
429/444; 429/454; 429/480; 429/483; 429/534 |
Current CPC
Class: |
H01M 8/0267 20130101;
H01M 8/04089 20130101; H01M 8/04074 20130101; H01M 8/241 20130101;
H01M 8/1004 20130101; H01M 2008/1095 20130101; H01M 8/242 20130101;
H01M 4/86 20130101; Y02E 60/50 20130101; H01M 8/0258 20130101; H01M
8/2457 20160201; H01M 8/1007 20160201 |
Class at
Publication: |
429/040 ;
429/030; 429/044; 429/026; 429/038; 429/013 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 8/10 20060101 H01M008/10; H01M 4/94 20060101
H01M004/94; H01M 8/04 20060101 H01M008/04; H01M 8/02 20060101
H01M008/02 |
Claims
1. A membrane electrode assembly for improved fuel cell performance
comprising: an ion-exchange membrane, an anode positioned on one
side of the membrane; and a cathode positioned on the opposite side
of the membrane, most of the area of the cathode opposing that of
the anode, wherein a portion of the cathode extends outside of the
anode area.
2. The membrane electrode assembly of claim 1 wherein the anode
comprises an anode diffusion layer coated with a catalyst layer and
the cathode comprises a cathode diffusion layer coated with a
catalyst layer.
3. The membrane electrode assembly of claim 1 wherein the
ion-exchange membrane covers substantially the entire area of the
anode.
4. The membrane electrode assembly of claim 1 wherein the
ion-exchange membrane covers substantially the entire area of the
cathode.
5. A fuel cell comprising the membrane electrode assembly of claim
1, wherein the fuel cell has an oxidant inlet and outlet and the
portion of the cathode extending outside of the anode area is on
the oxidant outlet side of the fuel cell.
6. The fuel cell of claim 5 wherein the membrane electrode assembly
is interposed between two flow field plate assemblies, each
comprising an internal coolant flow field, and wherein the area of
each internal coolant flow field extends outside of the anode area
on the oxidant outlet side of the fuel cell to cover substantially
the entire area of the cathode.
7. A fuel cell stack comprising at least one of the fuel cell
according to claim 5 or claim 6.
8. A fuel cell system comprising the fuel cell stack of claim 7,
wherein any hydrogen leaked from the anode side to the cathode side
of the at least one fuel cell is recombined with the oxygen on the
cathode side at a point before the oxidant outlet.
9. A method of improving the performance of a fuel cell comprising
an ion-exchange membrane, the method comprising arranging an anode
on one side of the membrane and a cathode on the opposite side of
the membrane, such that most of the area of the cathode opposes
that of the anode, a portion of the cathode extends outside of the
anode area, and any hydrogen leaked from the anode side of the fuel
cell to the cathode side is substantially recombined with the
oxygen on the cathode side at a point before an oxidant outlet of
the fuel cell.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119(e) of U.S. Provisional Patent Application No. 60/639,665 filed
Dec. 28, 2004. This provisional application is incorporated herein
by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a membrane electrode
assembly for improved fuel cell performance, and, more
specifically, to a membrane electrode assembly having a structure
that enables the recombining of any hydrogen leaked from the anode
side to the cathode side with the oxygen on the cathode side at a
point before the oxidant outlet.
[0004] 2. Description of the Related Art
[0005] Electrochemical fuel cells convert reactants, namely fuel
and oxidant, into electric power through the electrochemical
reactions that take place within the fuel cell. One type of fuel
cell that has been used for automotive and other industrial
applications because of its low operation temperature (around
80.degree. C.) is the solid polymer fuel cell. Solid polymer fuel
cells employ a membrane electrode assembly ("MEA") that includes an
ion exchange membrane disposed between two electrodes that carry a
certain amount of catalyst at their interface with the
membrane.
[0006] The catalyst is typically a precious metal composition
(e.g., platinum metal black or an alloy thereof) and may be
provided on a suitable support (e.g., fine platinum particles
supported on a carbon black support). A catalyst is needed to
induce the electrochemical reactions within the fuel cell. The
electrodes may also comprise a porous, electrically conductive
substrate that supports the catalyst layer and that is also
employed for purposes of electrical conduction, and/or reactant
distribution, thus serving as a fluid diffusion layer.
[0007] The MEA may be manufactured, for example, by bonding
together the catalyst-coated anode fluid diffusion layer, the
ion-exchange membrane and the catalyst-coated cathode fluid
diffusion layer under the application of heat and pressure. Another
method involves coating the catalyst layers directly onto the
ion-exchange membrane to form a catalyst-coated membrane and then
bonding the fluid diffusion layers thereon. The ion-exchange
membranes of particular interest are those prepared from
fluoropolymers that contain pendant sulfonic acid functional groups
and/or carboxylic acid functional groups. A typical
perfluorosulfonic acid/PTFE copolymer membrane can be obtained from
DuPont Inc. under the trade designation Naflon.RTM..
[0008] The MEA is typically disposed between two plates to form a
fuel cell assembly. The plates act as current collectors and
provide support for the adjacent electrodes. The assembly is
typically compressed to ensure good electrical contact between the
plates and the electrodes, in addition to good sealing between fuel
cell components. In operation, the output voltage of an individual
fuel cell under load is generally below one volt. Therefore, in
order to provide greater output voltage, numerous cells are usually
stacked together and are connected in series to create a higher
voltage fuel cell stack. In a fuel cell stack, a plate may be
shared between adjacent fuel cell assemblies, in which case the
plate also serves as a separator to fluidly isolate the fluid
streams of the two adjacent fuel cells. In a fuel cell, these
plates on either side of the MEA may incorporate flow fields for
the purpose of directing reactants across the surfaces of the fluid
diffusion electrodes or electrode substrates. The flow fields
comprise fluid distribution channels separated by landings. The
channels provide passages for the distribution of reactant to the
electrode surfaces and also for the removal of reaction products
and depleted reactant streams. The landings act as mechanical
supports for the fluid diffusion layers in the MEA and provide
electrical contact thereto. Since, during operation, the
temperature of the fuel cell may increase considerably and needs to
be controlled within permissible limits, flow field plates may also
include channels for directing coolant fluids along specific
portions of the fuel cell.
[0009] During normal operation of a solid polymer fuel cell, fuel
is electrochemically oxidized at the anode catalyst, typically
resulting in the generation of protons, electrons, and possibly
other species depending on the fuel employed. The protons are
conducted from the reaction sites at which they are generated,
through the ion-exchange membrane, to electrochemically react with
the oxidant on the cathode side. The electrons travel through an
external circuit providing useable power and then react with the
protons and oxidant at the cathode catalyst to generate water
reaction product.
[0010] A broad range of reactants can be used in solid polymer fuel
cells and may be supplied in either gaseous or liquid form. For
example, the oxidant stream may be substantially pure oxygen gas or
a dilute oxygen stream such as air. The fuel may be, for example,
substantially pure hydrogen gas, a gaseous hydrogen-containing
reformate stream, or an aqueous liquid methanol mixture in a direct
methanol fuel cell.
[0011] The membrane separates the reactant streams (fuel and
oxidant). Reactant isolation is very important because hydrogen and
oxygen are particularly reactive with each other. Therefore the
leakage of the reactants to the outside of the fuel cell has a very
negative impact on the fuel cell stack safety, performance and
longevity. If the membrane is defective (e.g., has a hole),
internal reactant transfer leaks may occur causing a
lifetime-limiting failure mode for the fuel cell stack. The way
this problem has been dealt with in the past is by designing fuel
cell systems to run with the fuel pressure on the anode side being
higher than the air pressure on the cathode side. This is done to
prevent air leaking into the anode side, which causes the cell go
into a fuel starvation mode. Fuel starvation can lead to cell
reversals, unit cell damage, MEA shorting and possible combustion
in the stack. MEAs are much less tolerant to fuel starvation than
to air starvation.
[0012] When the fuel cell system runs in a slight fuel overpressure
mode, hydrogen may leak from the anode side to the cathode side
through one or more holes in a defective or worn-out membrane.
Experimental tests have shown that, if the internal transfers do
not occur close to the air outlet end of the MEA, hydrogen will be
present at the cathode outlet only after the cell voltage has
collapsed to near-zero. To prevent this situation, the fuel cell
stack may be connected to a device for monitoring the voltages of
individual cells that will shut down the system and isolate the
fuel supply in the event of non-recoverable low cells. Tests have
shown that if the hydrogen internal leaks occur near the air
outlet, hydrogen is undesirably present in the cathode exhaust even
if the cell does not drop into complete air starvation mode.
[0013] One method to address issues associated with external
hydrogen leaks coming, for example, from the fuel processing
subsystem of a fuel cell system is to contain the leaks within a
housing. Such a housing may be provided with a recombiner that
catalytically converts hydrogen and oxygen into water, as disclosed
in U.S. Patent Application Publication No. 2003/0082428.
[0014] In addition, published Japanese Patent Application No.
2004146250 describes a membrane electrode assembly comprising glue
lines provided between the membrane and the electrodes to seal off
the fuel passage and the oxidant passage. The cathode has a larger
area than the anode such that it supports the entire membrane
surface to prevent any stress damage to the membrane. The anode
catalyst layer and the cathode catalyst layer have substantially
the same area. Although this application addresses the problem of
reactant mixing at the fuel cell inlet and outlet, it does not
address the problem of internal hydrogen transfer leaks through a
defective or worn-out membrane.
[0015] Accordingly, although there have been advances in the field,
there remains a need in the art for improved fuel cells,
particularly relating to internal hydrogen transfer leaks that may
occur near the oxidant outlet.
BRIEF SUMMARY OF THE INVENTION
[0016] A membrane electrode assembly comprises an ion exchange
membrane, an anode positioned on one side of the membrane and a
cathode positioned on the other side of the membrane, wherein most
of the area of the cathode opposes that of the anode.
[0017] A portion of the cathode extends outside of the anode area
such that any hydrogen leaked from the anode side of the fuel cell
to the cathode side due to any defects (e.g., holes) existing in
the membrane is recombined with the oxygen on the cathode side. The
anode and the cathode comprise a catalyst layer. The catalyst layer
may be deposited directly on the membrane or on a fluid diffusion
layer.
[0018] The membrane electrode assembly is part of a fuel cell
having an oxidant inlet and outlet. In a specific embodiment, the
portion of the cathode extending outside of the anode area is on
the oxidant outlet side of the fuel cell so that substantially all
the hydrogen leaked from the anode side to the cathode side is
recombined with oxygen on the cathode side before it reaches the
oxidant outlet, and thus substantially no hydrogen is present in
the oxidant stream exhausted from the fuel cell. The membrane
electrode assembly is interposed between two flow field plate
assemblies, each comprising an internal coolant flow field, and the
area of the coolant flow field extends outside of the anode area on
the oxidant outlet side of the fuel cell to cover substantially the
entire area of the cathode.
[0019] These and other aspects of the invention will be evident
upon reference to the following detailed description and attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a cross-sectional view of a fuel cell assembly as
known from the prior art.
[0021] FIG. 2 is an enlarged view of a detail of the cross-section
depicted in FIG. 1, showing a hole in the ion exchange
membrane.
[0022] FIG. 3 is a diagram showing a comparison of the experimental
test results in the case of a fuel cell with hydrogen transfer
leaks occurring near the oxidant inlet and outlet.
[0023] FIG. 4 is a cross-sectional view of the fuel cell according
to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present detailed description is generally directed
toward a membrane electrode assembly that comprises a cathode
extending outside of the anode area such that any hydrogen leaked
from the anode side of the fuel cell to the cathode side is
recombined with oxygen on the cathode side, thus reducing or
eliminating hydrogen in the oxidant stream exhausted from the fuel
cell.
[0025] FIG. 1 illustrates a conventional fuel cell assembly. For
simplicity, a single cell from a fuel cell stack is represented. It
is to be understood that this represents a repeating unit of the
fuel cell stack. The fuel cell 1 includes a membrane electrode
assembly (MEA) 2 comprising an ion-exchange membrane 3, an anode 4
and a cathode 5. The anode 4 comprises a catalyst layer 6 and may
also comprise a fluid diffusion layer 7. The cathode 5 comprises a
catalyst layer 8 and may also comprise a fluid diffusion layer 9.
The catalyst layer may be deposited directly on the membrane or may
be deposited on the fluid diffusion layers. The MEA may be
manufactured, for example, by bonding together the catalyst coated
anode fluid diffusion layer, the ion-exchange membrane and the
catalyst cathode fluid diffusion layer under the application of
heat and pressure. Another method involves coating the catalyst
layers directly onto the ion-exchange membrane to form a
catalyst-coated membrane and then bonding the fluid diffusion
layers thereon.
[0026] The MEA is typically interposed between two separator plate
assemblies 10 and 11, which are impermeable to the reactant fluid
streams. The MEA together with the separator plate assemblies form
the fuel cell assembly. The separator plate assemblies include flow
field channels 12 for directing reactants across one surface of
each electrode. The fuel flow field channels are fluidly connected
to each other to form a fuel stream 13 that is directed from the
fuel inlet of the fuel cell to the fuel outlet. Similarly, the
oxidant flow field channels are fluidly connected to each other to
form an oxidant stream 14 that is directed from the oxidant inlet
of the fuel cell to the oxidant outlet. Fuel cells are run with a
slight fuel overpressure compared to the oxidant pressure to
prevent fuel starvation due to the oxidant leaking to the anode
side. Fuel starvation has more negative effects on the stack than
oxidant starvation, leading to cell reversals, unit cell damage,
MEA shorting and possible combustion in the stack.
[0027] Each of the separator plate assemblies 10 and 11 also
includes coolant channels 15 that form a internal coolant flow
field through which coolant circulates generating a coolant stream
16 that cools down the fuel cell and helps keeping the temperature
of the stack within a permissible range (around 80.degree. C.).
[0028] The ion-exchange membrane may be prepared from
fluoropolymers containing pendant sulfonic acid functional groups
and/or carboxylic acid functional groups. A typical
perfluorosulfonic acid/PTFE copolymer membrane can be obtained from
DuPont Inc. under the trade designation Nafion.RTM.. Referring to
FIG. 2, if the membrane 3 has a defect 17, an internal transfer
leak occurs between the fuel stream 13 and the oxidant stream 14
since the pressure on the fuel side is higher than the pressure on
the cathode side. Such an internal transfer leaks limit the
lifetime of the fuel cell and have a negative impact on the
operation and performance of the fuel cell.
[0029] Experimental tests have shown that about 5% hydrogen present
at the oxidant inlet of the fuel cell can be completely recombined
before the leaked hydrogen reaches the oxidant outlet at 50 A. A
maximum of 30% hydrogen present at the oxidant inlet of the fuel
cell can be completely recombined in open circuit voltage (OCV)
conditions. Once all the available oxygen supplied to the fuel cell
is used to support the stack current and the hydrogen recombination
into water, the fuel cell goes into air starvation mode, the cell
voltage drops to near zero and hydrogen begins to be present in the
cathode exhaust stream.
[0030] Experimental tests have shown that, unlike the case of
hydrogen leaks occurring at the oxidant inlet, if the hydrogen
transfer leak occurs near the oxidant outlet, hydrogen is measured
at the oxidant outlet before the fuel cell drops into complete air
starvation. In this case, hydrogen is measured at the air outlet
almost immediately as the pressure differential increases above 0
bar as shown in FIG. 3. The relationship between the concentration
of the hydrogen present at the oxidant outlet and the pressure
differential between the fuel and the oxidant is approximately
linear with the outlet hydrogen concentration increasing gradually
as the pressure differential increases. As shown in FIG. 3, when
the internal hydrogen transfer leak occurs near the oxidant outlet,
the fuel cell voltage drops but does not become unstable and does
not fall to zero. Such an internal leak will not be detected by the
standard cell voltage monitoring device of the fuel cell system,
and therefore it is preferable to prevent the negative effects of
the hydrogen leaks occurring near the cathode outlet before
hydrogen leaks to the outside of the fuel cell.
[0031] A fuel cell 18 of the present invention, depicted in FIG. 4,
comprises MEA 19 comprising an ion-exchange membrane 20, an anode
21 and a cathode 22. The anode 21 comprises a catalyst layer 23 and
may also comprise a fluid diffusion layer 24. The cathode 22
comprises a catalyst layer 25 and may also comprise a catalyst
fluid diffusion layer 26. The MEA is typically interposed between
two separator plate assemblies 27 and 28 provided with reactant
flow field channels 29. The anode flow field channels are fluidly
connected to each other to form a fuel stream 30 flowing from the
fuel cell inlet of the fuel cell to the fuel outlet. Similarly, the
oxidant flow field channels are fluidly connected to each other to
form an oxidant stream 31 that is directed from the oxidant inlet
of the fuel cell to the oxidant outlet. The plate assemblies are
also provided with coolant channels 32 that form an internal
coolant flow field through which coolant circulates generating a
coolant stream 33.
[0032] As shown, most of the area of the cathode is opposing that
of the anode. The area where the anode and cathode overlap is
referred to as the "active area" 34, and is the area of the fuel
cell that enables the catalyst induced electrochemical reactions
between fuel and oxidant to generate electrical energy. A portion
35 of the cathode, referred to as the "hydrogen recombination
area", extends outside of the anode area and therefore outside of
the active area of the fuel cell. The length of portion 35 will
differ between embodiments of the present invention as a function
of reactant flow and hydrogen concentration in the cathode oxidant
stream 31. In certain embodiments, the length of portion 35 is
about 1.5 to 10% of the length of active area 34. If an internal
hydrogen leak occurs near the oxidant outlet due to a defect in the
membrane, substantially all of the leaked hydrogen will be
recombined beyond the active area of the fuel cell, and within the
hydrogen recombination area, before it reaches the oxidant
outlet.
[0033] Membrane 20 may cover substantially the entire area of the
anode (as shown in FIG. 4) or may extend outside of the anode area
and cover substantially the entire area of the cathode. In either
embodiment, membrane 20 serves to prevent reactant mixing and
short-circuiting.
[0034] The hydrogen recombination reactions that take place outside
of the active area generate heat and have a negative impact on the
thermal balance of the fuel cell. To address this issue, the
coolant flow field may be extended beyond the active area of the
fuel cell so that the coolant channels 32 cover the entire area of
the extended cathode (i.e., including the hydrogen recombination
area).
[0035] By extending the cathode and the corresponding coolant flow
field, negative effects of internal hydrogen transfer leaks
occurring near the oxidant outlet, that can go easily undetected by
voltage monitoring devices, are prevented. Any hydrogen that may be
present in the cathode exhaust is caused only by the internal
hydrogen transfer leaks occurring at the oxidant inlet and only
after the cell voltage has dropped to near-zero as indicated by the
fuel cell system voltage monitoring device. Consequently, the stack
will drop in performance before a flammability hazard exists in the
oxidant exhaust. In the event of non-recoverable low cells the fuel
cell system will shut down.
[0036] While particular steps, elements, embodiments and
applications of the present invention have been shown and
described, it will be understood, of course, that the invention is
not limited thereto since modifications may be made by persons
skilled in the art, particularly in light of the foregoing
teachings. It is therefore contemplated by the appended claims to
cover such modifications as incorporate those steps or elements
which come within the spirit and scope of the invention.
* * * * *